Under various flight conditions, ice may accumulate on aircraft surfaces such as leading edges, wings, tailplanes, and vertical stabilizers as an aircraft flies through a cloud containing super-cooled water droplets. Super-cooled water is water that is below freezing, but still a liquid. Normally, super-cooled water would turn to ice at 0° C., but there are no contaminants or nuclei from which ice crystallization freezing can start. When the aircraft flies through the super-cooled water droplets, the aircraft becomes a droplet nucleus, allowing the super-cooled water to freeze on an aircraft surface. This process is known as accretion. Super-cooled water droplets often occur in stratiform and cumulus clouds.
Airframe icing problems are generally caused by ice modifying airflow over ice covered aerodynamic lift surfaces such as a wing or tailplane. When ice accretes on aerodynamic lift surfaces, airflow modification changes aerodynamics of the aerodynamic lift surfaces by modifying their shape and surface roughness. Aerodynamic effects of icing on an aerodynamic lift surface are a function of the ice quantity, shape, and location, but typically increase drag and decrease lift. A composite effect of the increase in drag and decrease in lift is a degradation of aircraft flight dynamics.